NUMBERS

If your writing contains numbers, the general rule is to spell out in
letters all the numbers from zero to nine and use numerals for larger
numbers; but there are exceptions. If what you’re writing is full of
numbers and you’re doing math with them, stick with numerals.
Approximations like “about thirty days ago” and catch-phrases like “his
first thousand days” are spelled out. Large round numbers are often
rendered thus: “50 billion sold.” With measurements, use numerals: “4
inches long.” Never start a sentence with a numeral. Either spell out
the number involved or rearrange the sentence to move the number to a
later position.